In my near 50 years of exploring the science of audio I know of no research that addressed the "ideal" elevation of loudspeakers. From a technical point of view, one could argue that the direct sound arriving at a listener should be the best that a loudspeaker is capable of, which means the performance on the "reference axis" - almost never defined by a manufacturer, but widely assumed to be the tweeter axis. So the tradition developed that ear level tweeters were necessary.
However, current generation loudspeakers that are well designed exhibit very little difference between 0 deg on axis anechoic, and the spatially averaged listening window - an average of nine curves at 10 deg increments, ranging from +/- 30 deg hor. and +/- 10 deg vertical. So in reality there is a fairly generous acceptable loudspeaker/listener orientation window. With well designed loudspeakers the small differences are well within one's ability of adapt, and may be less than manufacturing tolerances on most loudspeakers. In fairness it must be noted that there are some loudspeakers that are extremely directional, so there may well be only one listening axis. For those to be elevated a tilt must be introduced.
So, the real question relates to the perceived "soundstage" and how it fits into listener expectations. Frankly, I often felt over the years that conventional loudspeaker placement yielded an unrealistically low presentation, but one of course adapts to that and it becomes the "norm". Until, one experiments. With visual images being elevated there is a justification for elevating the loudspeakers, but even then, the ventriloquism effect is so powerful that we don't notice that for much of most movies all of the consequential sounds come from the center channel, and it can be above or below the screen without being noticed - even in cinemas unless one sits close to the front.
As I, and friends, experience my current system the consensus from stereo listening is that inverting the Salon2s results in a very slightly elevated soundstage, which is very open. It is an attractive presentation - for me. For movies it is right on target. Such an arrangement also liberates valuable floor space. In recording studios it is common for monitor speakers to be slightly elevated, and tilted if necessary. So-o-o-o if one believes in the circle of confusion, there is precedent :-)